1. Field of the Invention
This invention relates to a control system for a vehicle safety device.
2. Discussion of the Related Art
As disclosed in Japanese Laid-Open (Kokai) Patent Application No. 55031/74 and Japanese Laid-Open Utility Model Application No. 5371/90, a system for controlling a vehicle safety device, such as an air bag, is already known. This control system comprises acceleration detecting means for detecting a negative acceleration (i.e., deceleration) of a vehicle, integrating means for integrating the deceleration outputted from the acceleration detecting means, collision judgment means for comparing the integral value from the integrating means with a threshold level so as to judge whether or not a collision has occurred, and for outputting an activation instruction signal when the collision judgment means judges that the collision has occurred, and a drive circuit responsive to the activation instruction signal from the collision judgment means so as to supply electric current to a squib of the air bag to thereby expand the air bag.
In the above control system, the time period from the time of start of the deceleration caused by the collision to the time of completion of the expanding of the air bag is determined by the sum of the following first and second time periods. The first time period begins from the time of start of the deceleration, and ends at the time when the activation instruction signal for supplying the current to the squib is outputted when the integral value of the deceleration reaches the threshold level. The second time period begins from the time of supply of the current to the squib in response to the activation instruction signal, and ends at the time of completion of the expanding of the squib.
When the vehicle heavily collides (in other words, when an abrupt deceleration occurs for a short time period), the time period from the time of start of the deceleration to the time when the passengers are forced forwardly due to an inertia force is short. Therefore, in this case, it is necessary to expand the air bag early. In the above control system, when the deceleration is large, the integral value of the deceleration reaches the threshold level early, and therefore the above first time period can be shortened. However, since the above second time period is constant, there remains a possibility that the sum of the first and second time periods can not be shortened sufficiently.
On the other hand, when the collision is rather mild (in other words, when the deceleration occurs for a relatively long time period), there is no risk that the passengers impinge on a steering wheel, etc., because the inertia force exerted on the passengers is small. Therefore, in this case, it is necessary to prevent the air bag from being expanded. However, even when the deceleration is small, the integral value increases with time, and therefore there is a possibility that the integral value finally reaches to the threshold level, thereby causing an inadvertent activation of the air bag.
This difficulty is attributable to the fact that the threshold level for judging the collision is constant. One of the inventors of the present invention filed a U.S. patent application Ser. No. 473,561 on Jan. 19, 1990 which is directed to the technique of adjusting a threshold level for collision judgment from an aspect different from that of the present invention. Japanese Published Application No. 503531/88 (corresponding to International Application PCT/DE87/00078 including the United States as one of the designated countries), and U.S. Pat. Nos. 4,638,179, 4,836,024 and 4,497,025 are listed as the relevant prior art.